A direct memory access (DMA) engine moves a portion of data from a source device to a destination device autonomously from CPU control. An individual DMA transfer is configured by setting up the DMA engine to fetch a descriptor from memory, which contains a description of the transfer to be performed. A DMA descriptor normally contains: the source address of the data to be transferred, the destination address for that data, a number of control parameters, and often a link to a subsequent transfer for the DMA engine to process once complete. Usually a DMA engine will be constructed of a number of independent contexts processing transfers in parallel, each context having dedicated hardware.
One potential source of data for a DMA transfer is an interface with a peripheral slave device. Commonly, when a DMA engine reads data from a slave device, the data must be interrogated to determine its relevance. This can involve interrupting the usual CPU flow to process the received data, which in real-time software can adversely affect performance. Alternatively all received data can be copied into a memory buffer to be processed later. Depending on the activity across the interface this can result in large amounts of data to process, increasing the overall CPU load.
It would be advantageous to reduce the CPU overhead required to find the desired data.